This invention relates generally to a panel used for attenuating sound frequencies and more particularly, but not by way of limitation, to a cellular honeycomb sandwich construction for installation in an engine nacelle or other applications requiring sound absorption.
Heretofore, advanced technology sound attenuating panels in an engine nacelle have been constructed primarily using a double layer of honeycomb core with a septum bonded between the two layers. Facing material is then bonded on both sides of the double layer forming a core sandwich. This type of construction requires four bond lines which degrades the strength of the sandwich structure. Also, it is impossible to align the individual cells of the adjacent layers. Further, heat transfer is less efficient and varies due to the septum bond lines and the misalignment of the individual cells.
In the fabrication of a core sandwich where a septum is provided having various depths within the individual cells, complex machining and assembly techniques are required. This is not only expensive, but time consuming.
A prior art acoustical structure is used wherein a single layer of honeycomb core is provided with an intermittent zone in the core which is crushed to form a second barrier therein. Also, there is a single layer honeycomb core structure wherein the individual cells are filled with foam plugs to a single cell depth. This structure does not provide for varying depth cells. An additional sound absorption honeycomb core structure provides a cellular array cut obliquely so that the effective lengths of the cells vary along the plane of the cut. None of the prior art core structures provide the advantages of the subject invention in retarding jet engine noise.